Existing methods of torsion measurement for rotating shafts rely on either contact sensors or magnetic pickup. In these measurements systems, sensors detect phase displacement proportional to the torsion between two or more triggers placed at distinct locations on the shaft. A primary disadvantage of contact torsion sensors is that they suffer wear due to friction. Other disadvantages are that such methods are difficult to retrofit onto existing machines, and they have a limited angular resolution. An example of the magnetic pickup torsion sensing system, described by Troeder (U.S. Pat. No. 4,450,729), utilizes differential magnetic resistor sensors for non-contact measurement. This device still requires retrofitting of toothed wheels onto the shaft. In an alternate embodiment, Troeder et al described inductive sensors instead of magnetic resistors.
When installed upon manufacture of the shaft, the non-contact magnetic pickup or inductive sensor detection systems are practical and fairly reliable at moderate rotation speeds. However, toothed wheels, as described above, are difficult to manufacture so that they are identical to each other, therefore toothed wheel-based devices have limited resolution. Magnetic pick-up detectors are also limited in very high speed applications, and spurious signals may be produced due to operating environment conditions such as electromagnetic interference and high temperatures. Inductive sensors suffer a reduction in signal at very low frequencies based on Faraday's Law, and will also be affected by electromagnetic noise, but are less sensitive to high temperatures.
It is desirable, then, to have a high resolution device which would be easily retrofitted onto existing equipment without substantial modification, and is capable of accurate measurement for both high and low speed applications. Other desirable features of such a device would have high tolerance of adverse environmental conditions such as high temperature, radioactivity, electromagnetic and RF noise, and smoke or fog. It is to this end that the present invention is directed.